White flame-resistant UV-stable thermoformable film made from a crystallizable thermoplastic, a method for production and the use thereof

ABSTRACT

The invention relates to a white, flame-resistant, UV-stable, thermoformable, oriented film made from a crystallizable thermoplastic, the thickness of which lies in the range of from 10 μm to 350 μm. Said film comprises at least one white pigment, a flame-proofing agent and a UV absorber and is characterized by good stretchability and thermoformability, by good optical and mechanical properties and an economical production. The invention further relates to a method for the production of said film and the use thereof.

The invention relates to a white, flame-retardant, UV-resistant,thermoformable, oriented film made from a crystallizable thermoplastic,the thickness of the film being in the range from 10 to 350 μm. The filmcomprises at least one white pigment and one flame retardant and one UVabsorber and has good orientability and thermoformability, and very goodoptical and mechanical properties, and can be produced cost-effectively.The invention further relates to the use of this film and to a processfor its production.

BACKGROUND OF THE INVENTION

White, oriented films made from crystallizable thermoplastics with athickness of from 10 to 350 μm are well known.

These films do not comprise UV absorbers of any kind as lightstabilizers and do not comprise flame retardants of any kind, andtherefore neither the films nor the items produced from them aresuitable for outdoor applications which demand fire protection or flameretardancy. The films do not pass the fire tests to DIN 4102 Part 2 andPart 1, or the UL 94 test. The films have inadequate thermoformability.

Even after a short time in outdoor applications, these films yellow andexhibit impairment of mechanical properties due to photooxidativedegradation by sunlight.

EP-A-0 620 245 describes films with improved heat resistance. Thesefilms comprise antioxidants which are suitable for scavenging freeradicals formed in the film and degrading any peroxide formed. However,that specification gives no proposal as to how the UV resistance ofthese films might be improved.

DE-A 2346 787 describes a flame-retardant polymer. Alongside thepolymer, the use of the polymer is also claimed for producing films orfibers.

The following shortcomings were apparent during production of films fromthis phospholane-modified polymer:

The polymer is very susceptible to hydrolysis and has to be verythoroughly predried. The polymer cakes during its drying by prior-artdryers, and it is impossible to produce a film except under the mostdifficult of conditions.

The films produced under extreme and uneconomic conditions embrittle onexposure to heat, i.e. the mechanical properties are severely impaireddue to substantial embrittlement, making the film unusable. Thisembrittlement occurs after as little as 48 hours of exposure to heat.

It was an object of the present invention to provide a white,flame-retardant, UV-resistant, thermoformable, oriented film with athickness of from 10-350 μm which not only can be producedcost-effectively and has good orientability and good mechanical andoptical properties, but in particular is flame retardant, does notembrittle on exposure to heat, is thermoformable, and has high UVresistance.

Flame retardancy means that in a fire test the white film complies withthe conditions of DIN 4102 Part 2 and in particular the conditions ofDIN 4102 Part 1, and can be allocated to construction materials class B2 and in particular B1 for low-flammability materials.

The film is also intended to pass the UL 94 test “Vertical Burning Testfor Flammability of Plastic Material”, permitting its classification inclass 94 VTM-0. This means that 10 seconds after removal of the Bunsenburner the film has ceased to burn, and after 30 seconds no glowing isobserved, and no drips are found to occur.

High UV resistance means that sunlight or other UV radiation causes no,or only extremely little, damage to the films, so that the films aresuitable for outdoor applications and/or critical indoor applications.In particular, after a number of years in outdoor applications the filmsare intended not to yellow, nor to exhibit any embrittlement or surfacecracking, nor to exhibit any impairment of mechanical properties. HighUV resistance therefore means that the film absorbs UV light and doesnot transmit light until the visible region has been reached.

Thermoformability means that the film can be thermoformed to givecomplex and large-surface-area moldings on commercially availablethermoforming machinery, without uneconomic predrying.

Examples of good optical properties include uniform coloration, highsurface gloss (>15), low light transmission (<70%), and also aYellowness Index unchanged from that of the flame-retardant andUV-modified film.

Good mechanical properties include high modulus of elasticity(E_(MD)>3200 N/mm²: E_(TD)>3500 N/mm²), and also good values for tensilestress at break (in MD >100 N/mm²; in TD >130 N/mm²).

Good orientability includes the capability of the film to give excellentorientation, both in a longitudinal direction and I transverse directionduring its production, without break-offs.

Cost-effective production includes the capability of the raw materialsor raw material components needed to produce the flame-retardant film tobe dried using industrial-standard dryers. It is important that the rawmaterials neither cake nor become thermally degraded. These prior-artindustrial dryers include vacuum dryers, fluidized-bed dryers, fixed-beddryers (tower dryers). These dryers operate at temperatures of from 100to 170° C., at which the flame-retardent polymers cake and have to bedug out, making film production impossible.

In the case of the vacuum dryer, which provides the mildest dryingconditions, the raw material traverses a temperature range from about 30to 130° C., under a vacuum of 50 mbar. Post-drying is then needed in ahopper at temperatures from 100 to 130° C. with a residence time of from3 to 6 hours. Here, too, this polymer cakes to an extreme extent.

BRIEF DESCRIPTION OF THE INVENTION

This object is achieved by means of a white thermoformable film with athickness in the range from 10 to 350 μm, which comprises acrystallizable thermoplastic principal constituent, and comprises atleast one white pigment, at least one UV absorber, and at least oneflame retardant, where expediently the UV absorber and according toinvention the flame retardant are fed directly as masterbatch during theproduction of the film.

DETAILED DESCRIPTION OF THE INVENTION

The white, flame-retardant, UV-resistant, thermoformable, oriented filmcomprises, as principal constituent, a crystallizable thermoplastic.Examples of suitable crystallizable or semicrystalline thermoplasticsare polyethylene terephthalate, polybutylene terephthalate, polyethylenenaphthalate, preferably polyethylene terephthalate.

According to the invention, crystallizable thermoplastics arecrystallizable homopolymers, crystallizable copolymers, crystallizablecompounded materials (mixtures), crystallizable recycled material, andother types of crystallizable thermoplastics.

The white film may be either a single-layer or a multilayer film. Thefilm may also have a coating of various copolyesters or adhesionpromoters.

According to the invention, the white film comprises a UV absorber and aflame retardant. The UV absorber is expediently fed directly during theproduction of the film by way of masterbatch technology, theconcentration of the UV stabilizer preferably being from 0.01 to 5% byweight, based on the weight of the layer of the crystallizablethermoplastic.

No embrittlement on brief exposure to heat means that after 100 hours ofa heat-conditioning procedure at 100° C. in a circulating-air oven thefilm or the molding exhibits no embrittlement nor any poor mechanicalproperties.

The film of the invention comprises at least one flame retardant, feddirectly during the production of the film by way of masterbatchtechnology, the concentration of the flame retardant being in the rangefrom 0.5 to 30.0% by weight, preferably from 1.0 to 20.0% by weight,based on the weight of the layer of the crystallizable thermoplastic.The ratio of flame retardant to thermoplastic maintained duringproduction of the masterbatch is generally in the range from 60:40% byweight to 10:90% by weight.

Typical flame retardants include bromine compounds, chloroparaffins, and10 other chlorine compounds, antimony trioxide, aluminum trihydrates,the halogen compounds being disadvantageous due to thehalogen-containing by-products produced. Another extreme disadvantage isthe low lightfastness of a film modified therewith, alongside theevolution of hydrogen halides in the event of a fire.

Examples of suitable flame retardants used according to the inventionare organophosphorus compounds, such as carboxyphosphinic acids,anhydrides of these, and dimethyl methylphosphonate. It is important forthe invention that the organophosphorus compound is soluble in thethermoplastic, since otherwise the optical properties required are notcomplied with.

Since the flame retardants generally have some susceptibility tohydrolysis, it can be advisable to add a hydrolysis stabilizer.

Hydrolysis stabilizers used are generally phenolic stabilizers, alkalimetal/alkaline earth metal stearates, and/or alkali metal/alkaline earthmetal carbonates, in amounts of from 0.01 to 1.0% by weight. It ispreferable to use amounts of from 0.05 to 0.6% by weight, in particularfrom 0.15 to 0.3% by weight, of phenolic stabilizers having a molar massabove 500 g/mol. Particularly advantageous compounds are pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxphenyl) propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

The white pigment is preferably fed by way of masterbatch technology,but may also be incorporated directly at the premises of the polymerproducer. The concentration of the white pigment is from 0.2 to 40% byweight, preferably from 0.5 to 25% by weight, based on the weight of thecrystallizable thermoplastic.

Preferred suitable white pigments are titanium dioxide, barium sulfate,calcium carbonate, kaolin, silicon dioxide, preferably titanium dioxideand barium sulfate.

The titanium dioxide particles may be composed of anatase or rutile,preferably predominantly of rutile, which has higher opacifying powerthan anatase.

In a preferred embodiment, the titanium dioxide particles are composedof at least 95% by weight of rutile. They may be prepared by aconventional process, e.g.: by the chloride process or the sulfateprocess. The amount of these in the base layer is from 0.3 to 25% byweight, based on the base layer, and the average particle size isrelatively small, preferably in the range from 0.10 to 0.30 μm.

Titanium dioxide of the type described does not produce any vacuolswithin the polymer matrix during the production of the film.

The titanium dioxide particles may have the type of covering usuallyused as a covering for TiO₂ white pigment in papers or paints to improvelightfastness, made from inorganic oxides.

TiO₂ is known to be photoactive. On exposure to UV radiation, freeradicals form on the surface of the particles. These free radicals canmigrate into the film-forming polymers, causing degradation reactionsand yellowing.

Particularly suitable oxides include the oxides of aluminum, silicon,zinc, or magnesium, and mixtures made from two or more of thesecompounds. TiO₂ particles with a covering made from two or more of thesecompounds are described by way of example in EP-A-0 044 515 and EP-A-0078 633. The coating may also comprise organic compounds having polarand non-polar groups. The organic compounds have to have adequatethermal stability during production of the film by extrusion of thepolymer melt. Examples of polar groups are —OH, —OR, —COOX (X═R, H, orNa, R=alkyl having from 1 to 34 carbon atoms). Preferred organiccompounds are alkanols and fatty acids having from 8 to 30 carbon atomsin the alkyl group, in particular fatty acids and primary n-alkanolshaving from 12 to 24 carbon atoms, and also polydiorganosiloxanes and/orpolyorganohydrosiloxanes, e.g. polydimethylsiloxane andpolymethylhydrosiloxane.

The coating for the titanium dioxide particles is usually composed offrom 1 to 12 g, in particular from 2 to 6 g, of inorganic oxides, andfrom 0.5 to 3 g, in particular from 0.7 to 1.5 g, of organic compounds,based on 100 g of titanium dioxide particles. The covering is applied tothe particles in aqueous suspension. The inorganic oxides may beprecipitated from water-soluble compounds, e.g. alkali metal nitrate, inparticular sodium nitrate, sodium silicate (waterglass), or silica, inthe aqueous suspension.

For the purposes of the present invention, inorganic oxides, such asAl₂O₃ or SiO₂, also include the hydroxides and their various stages ofdehydration, e.g. oxide hydrate, the precise composition and structureof which is not known. The oxide hydrates, e.g. of aluminum and/or ofsilicon, are precipitated onto the calcined and ground TiO₂ pigment, inaqueous suspension, and the pigments are then washed and dried. Thisprecipitation may therefore take place directly in a suspension such asthat produced within the production process after calcination followedby wet-grinding. The oxides and/or oxide hydrates of the respectivemetals are precipitated from the water-soluble metal salts within theknown pH range: for example, for aluminum use is made of aluminumsulfate in aqueous solution (pH below 4), and the oxide hydrate isprecipitated within the pH range from 5 to 9, preferably from 7 to 8.5,by addition of aqueous ammonia solution or sodium hydroxide solution. Ifthe starting material is waterglass solution or alkali metal aluminatesolution, the pH of the initial charge of TiO₂ suspension should bewithin the strongly alkaline range (pH above 8). The precipitation thentakes place within the pH range from 5 to 8, by addition of mineralacid, such as sulfuric acid. Once the metal oxides have beenprecipitated, the stirring of the suspension continues for from 15 minto about 2 h, aging the precipitated layers. The coated product isseparated off from the aqueous dispersion, washed, and dried at anelevated temperature, in particular at from 70 to 100° C.

Light, in particular the ultraviolet content of solar radiation, i.e.the wavelength region from 280 to 400 nm, induces degradation inthermoplastics, as a result of which their appearance changes due tocolor change or yellowing, and there is also an adverse effect onmechanical/physical properties.

Inhibition of this photooxidative degradation is of considerableindustrial and economic importance, since otherwise there are drasticlimitations on the applications of many thermoplastics.

Absorption of UV light by polyethylene terephthalates, for example,starts at below 360 nm, increases markedly below 320 nm, and is verypronounced at below 300 nm. Maximum absorption occurs at from 280 to 300nm.

In the presence of oxygen it is mainly chain cleavage which occurs,without any crosslinking. The predominant photooxidation products inquantity terms are carbon monoxide, carbon dioxide, and carboxylicacids. Besides the direct photolysis of the ester groups, considerationhas to be given to oxidation reactions which likewise form carbondioxide, via peroxide radicals.

In the photooxidation of polyethylene terephthalates there can also becleavage of hydrogen at the position α to the ester groups, givinghydroperoxides and decomposition products of these, and this may beaccompanied by chain cleavage (H. Day, D. M. Wiles: J. Appl. Polym. Sci16, 1972, p. 203).

UV stabilizers, i.e. light stabilizers which are UV absorbers, arechemical compounds which can intervene in the physical and chemicalprocesses of light-induced degradation. Carbon black and other pigmentscan give some protection from light. However, these substances areunsuitable for transparent films, since they cause discoloration orcolor change. The only compounds suitable for transparent matt films areorganic and organometallic compounds which produce no, or only extremelyslight, color or color change in the thermoplastic to be stabilized,i.e. those which are soluble in the thermoplastic.

For the purposes of the present invention, UV stabilizers suitable aslight stabilizers are those which absorb at least 70%, preferably 80%,particularly preferably 90%, of the UV light in the wavelength regionfrom 180 to 380 nm, preferably 280 to 350 nm. These are particularlysuitable if they are thermally stable in the temperature range from 260to 300° C., i.e. neither decompose nor give rise to release of gases.Examples of UV stabilizers suitable as light stabilizers are2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickel compounds,salicylic esters, cinnamic ester derivatives, resorcinol monobenzoates,oxanilides, hydroxybenzoic esters, and sterically hindered amines andtriazines, preference being given to the 2-hydroxybenzotriazoles and thetriazines.

The UV stabilizer(s) are preferably present in the outer layer(s). Thecore layer may also have UV stabilizer, if required.

It was highly surprising that the use of the abovementioned UVstabilizers in films gave the desired result. The skilled worker wouldprobably first have attempted to achieve a certain degree of UVresistance by way of an antioxidant, but would have found that the filmrapidly yellows on weathering.

In the knowledge that UV stabilizers absorb UV light and thereforeprovide protection, the skilled worker would be likely to have usedcommercially available stabilizers. He would then have observed that

the UV stabilizer has unsatisfactory thermal stability, and attemperatures of from 200 to 240° C. decomposes and gives rise to releaseof gases, and

large amounts (from about 10 to 15% by weight) of the UV stabilizer haveto be incorporated in order to absorb the UV light and thus preventdamage to the film.

At these high concentrations it would have been observed that the filmis yellow even just after it has been produced, with Yellowness Indices(YI) of around 25. It would also have been observed that the mechanicalproperties of the film have been adversely affected. Orientation wouldhave produced exceptional problems, such as

break-offs due to unsatisfactory strength, i.e. excessively low modulusof elasticity;

die deposits, causing profile variations;

roller deposits from the UV stabilizer, causing impairment of opticalproperties (defective adhesion, non-uniform surface);

deposits in stretching frames or heat-setting frames, dropping onto thefilm.

It was therefore more than surprising that even low concentrations ofthe UV stabilizer achieve excellent UV protection. It was verysurprising that, together with this excellent UV protection,

within the accuracy of measurement, the Yellowness Index of the film isunchanged from that of an unstabilized film;

there are no releases of gases, no die deposits, and no framecondensation, and the film therefore has excellent optical propertiesand excellent profile and layflat, and

the UV-resistant film has excellent stretchability, and can therefore beproduced in a reliable and stable manner on high-speed film lines atspeeds of up to 420 m/min.

It was more than surprising that the use of masterbatch technology andof appropriate predrying and/or precrystallization and, whereappropriate, use of small amounts of a hydrolysis stabilizer permit theproduction of a flame-retardant and thermoformable film with theproperty profile demanded in a cost-effective manner and without cakingin the dryer, and that the film does not embrittle on exposure to heatand does not fracture when creased. It was very surprising that togetherwith this excellent result and the required flame retardancy, and thethermoformability and high UV resistance

within the accuracy of measurement, the Yellowness Index of the film isnot adversely affected when compared with that of an unstabilized film;

there are no releases of gases, no die deposits, and no framecondensation, and the film therefore has excellent optical propertiesand excellent profile and layflat, and

the flame-retardant UV-resistant film has excellent stretchability, andcan therefore be produced in a reliable and stable manner on high-speedfilm lines at speeds of up to 420 m/min.

With this, the film is also cost-effective.

It was also surprising that a higher diethylene glycol content and/orpolyethylene glycol content and/or IPA content than that of standardthermoplastics permits cost-effective thermoforming of the films oncommercially available thermoforming plants, and gives the filmscapability for excellent reproduction of detail.

It is moreover very surprising that it is also possible to reuse theregrind produced from the films or from the moldings without adverselyaffecting the Yellowness Index of the film.

In one preferred embodiment, the white, flame-retardant film of theinvention comprises, as principal constituent, a crystallizablepolyethylene terephthalate having a diethylene glycol content of ≧1.0%by weight, preferably ≧1.2% by weight, in particular ≧1.3% by weight,and/or a polyethylene glycol content (PEG content) of ≧1.0% by weight,preferably ≧1.2% by weight, in particular ≧1.3% by weight, from 1 to 20%by weight of an organic phosphorus compound (dimethyl methylphosphonate)as flame retardant soluble in the polyethylene terephthalate, from 0.01to 5.0% by weight of a UV absorber selected from the group of the2-hydroxybenzotriazoles or the triazines and soluble in the PET, andfrom 0.5 to 25% by weight of titanium dioxide whose preferred particlediameter is from 0.10 to 0.50 μm, preferably a rutile-type titaniumdioxide. Instead of titanium dioxide, it is also possible to use bariumsulfate whose particle diameter is from 0.20 to 1.20 μm as whitepigment, the concentration being from 1.0 to 25% by weight. In onepreferred embodiment, it is also possible to use a mixture of thesewhite pigments, or a mixture of one of these white pigments with anotherwhite pigment.

In one particularly preferred embodiment, the film of the inventioncomprises from 0.01 to 5.0% by weight of2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl) oxyphenol of the formula

or from 0.01 to 5.0% by weight of2,2′-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethyl-butyl)phenolof the formula

In one preferred embodiment, it is also possible to use a mixture ofthese two UV stabilizers, or a mixture of at least one of these two UVstabilizers with other UV stabilizers, the total concentration of lightstabilizer preferably being from 0.01 to 5.0% by weight, based on theweight of crystallizable polyethylene terephthalate.

In the invention it is important for thermoformability that thecrystallizable thermoplastic has a diethylene glycol content (DEGcontent) of ≧1.0% by weight, preferably ≧1.2% by weight, in particular≧1.3% by weight, and/or a polyethylene glycol content (PEG content) of≧1.0% by weight, preferably ≧1.2% by weight, in particular ≧1.3% byweight, and/or an isophthalic acid content (IPA) of from 3 to 10% byweight.

The white, UV-resistant, thermoformable, flame-retardant film has thefollowing property profile:

surface gloss, measured to DIN 67530 (measurement angle 20°), is greaterthan 15, preferably greater than 20, and light transmittance L, measuredto ASTM D 1003, is less than 70%, preferably less than 60%, measured toASTM S 1003, this being surprisingly good for the UV resistance achievedin combination with the flame retardancy.

Standard viscosity SV (DCA) of the polyethylene terephthalate, measuredin dichloroacetic acid to DIN 53728 is from 600 to 1000, preferably from700 to 900.

The white polyethylene terephthalate film which comprises at least oneorganic white pigment, one UV stabilizer, and one flame retardant may beeither a single-layer film or a multilayer film.

In the multilayer embodiment, the film is built up from at least onecorner layer and from at least one outer layer, preference being givenin particular to a three-layer A-B-A or A-B-C structure.

For this embodiment it is important that standard viscosity and DEGcontent and/or PEG content of the polyethylene terephthalate of the corelayer are similar to those of the polyethylene terephthalate of theouter layer(s) adjacent to the core layer.

In a particular embodiment, the outer layers may also be composed of apolyethylene naphthalate homopolymer or of a polyethyleneterephthalate-polyethylene naphthalate copolymer, or of a compoundedmaterial.

Again in this embodiment, standard viscosity and DEG content and/or PEGcontent of the thermoplastics of the outer layers are similar to thoseof the polyethylene terephthalate of the core layer.

In the multilayer embodiment, the UV absorber is preferably present inthe outer layers. If required, UV absorber may also have been providedin the core layer.

In the multilayer embodiment, the white pigment and the flame retardantare preferably present in the core layer. However, if required, whitepigment and/or flame retardant may also have been provided in the outerlayers.

In another embodiment it is also possible for white pigment, flameretardant and UV absorber to be present in the outer layers. If requiredand if fire protection requirements are stringent, the core layer mayalso have what is known as a “base level” of flame retardant.

Unlike in the single-layer embodiment, the concentration of the whitepigment here, and of the flame retardant and of the UV stabilizer, isbased on the weight in the modified layer. Highly surprisingly,weathering tests to the ISO 4892 test specification using the Atlas C165Weather Ometer have shown that in order to achieve improved UVresistance for a three-layer film it is fully sufficient for the outerlayers of thickness of from 0.5 to 2 μm to be provided with UVstabilizers. Fire tests to DIN 4102 Part 1 and Part 2, and also the UL94 test have equally surprisingly shown that compliance of the film ofthe invention with the requirements extends to the range of thicknessfrom 5 to 300 μm.

The flame-retardant, UV-resistant, thermoformable, multilayer filmsproduced using known coextrusion technology are therefore of greateconomic interest when compared with monofilms provided with UVstabilizers and flame retardants throughout, since markedly lessadditives are needed for comparable flame retardancy and UV resistance.

At least one side of the film may also have been provided with ascratch-resistant coating, with a copolyester, or with an adhesionpromoter.

Weathering tests have shown that even after from 5 to 7 years of outdooruse (extrapolated from the weathering tests) the flame-retardantUV-resistant films of the invention generally exhibit no increasedyellowing, no embrittlement, no loss of surface gloss, no surfacecracking, and no impairment of mechanical properties.

The results of measurements indicate that the film of the invention orthe molding does not embrittle when exposed to heat at 100° C. over aprolonged period. This result is attributable to the synergistic actionof appropriate precrystallization, predrying, masterbatch technology,and modification with UV stabilizer.

The film can be thermoformed without predrying, and can therefore beused to produce complex moldings.

The thermoforming process generally encompasses the steps of predrying,heating, molding, cooling, demolding, and heat-conditioning.Surprisingly, during the thermoforming process it was found that thefilms of the invention can be thermoformed without prior predrying. Thisadvantage over thermoformable polycarbonate films or thermoformablepolymethacrylate films, which require predrying times of from 10 to 15hours, at temperatures of from 100 to 120° C., depending on thickness,drastically reduces the costs of the forming process.

The following process parameters for the thermoforming process werefound:

Step of process Film of invention Predrying not required Temperature ofmold °C. from 100 to 160 Heating time <5 sec per 10 μm of film thicknessFilm temperature during from 160 to 220 thermoforming °C. Possibleorientation factor from 1.5 to 2.0 Reproduction of detail good Shrinkage(%) <1.5

The film of the invention or the molding produced therefrom can moreoverbe recycled without difficulty and without pollution of the environment,and without loss of mechanical properties, and is therefore suitable foruse as short-lived advertising placards, for example, for theconstruction of exhibition stands, or for other promotional items wherefire protection and thermoformability is desired.

An example of a method for producing the white, flame-retardant,thermoformable, UV-resistant film of the invention is the extrusionprocess on an extrusion line.

According to the invention, the flame retardant is added by way ofmasterbatch technology. The flame retardant is fully dispersed in acarrier material. Carrier materials which may be used are thethermoplastic itself, e.g. the polyethylene terephthalate, or else otherpolymers compatible with the thermoplastic.

According to the invention, the UV stabilizer and the white pigment maybe fed before the material leaves the producer of the thermoplasticpolymer, or during the production of the film, into the extruder.

DEG content and/or PEG content of the polyethylene terephthalate are setat the premises of the polymer producer during the polycondensationprocess.

Addition of the white pigment and of the UV stabilizer by way ofmasterbatch technology is particularly preferred. The UV stabilizer and,respectively, the white pigment is fully dispersed in a solid carriermaterial. Carrier materials which may be used are the thermoplasticitself, e.g. the polyethylene terephthalate, or else other polymerssufficiently compatible with the thermoplastic.

It is important in masterbatch technology that the grain size and thebulk density of the masterbatch are similar to the grain size and thebulk density of the thermoplastic, thus permitting uniform distributionand, with this, uniform UV resistance.

The polyester films may be produced by known processes from a polyester,where appropriate with other polymers, with the flame retardant, withthe white pigment, with the UV absorber, and/or with other conventionaladditives in conventional amounts from 1.0 to not more than 30% byweight, either in the form of a monofilm or else in the form ofmultilayer, where appropriate coextruded films with surfaces ofidentical or different nature, for example pigment being present in onesurface but no pigment being present in the other surface. It is alsopossible for one or both surfaces of the film to be provided with aconventional functional coating by known processes.

It is important for the invention that the masterbatch which comprisesthe flame retardant and, where appropriate, the hydrolysis stabilizer,is precrystallized or predried. This predrying includes progressiveheating of the masterbatch at subatmospheric pressure (from 20 to 80mbar, preferablyfrom 30 to 60 mbar, in particular from 40 to 50 mbar),with stirring, and, where appropriate, post-drying at a constantelevated temperature, again at subatmospheric pressure. The masterbatchis preferably charged at room temperature from a feed vessel in thedesired blend with the polymers of the base and/or outer layers and,where appropriate, with other raw material components, batchwise in avacuum dryer which during the course of the drying time or residencetime traverses a temperature profile from 10 to 160° C., preferably from20 to 150° C., in particular from 30 to 130° C. During the residencetime of about 6 hours, preferably 5 hours, in particular 4 hours, theraw material mixture is stirred at from 10 to 70 rpm, preferably from 15to 65 rpm, in particular from 20 to 60 rpm. The resultantprecrystallized or predried raw material mixture is post-dried for from2 to 8 hours, preferably from 3 to 7 hours, in particular from 4 to 6hours, in a downstream vessel, likewise evacuated, at from 90 to 180°C., preferably from 100 to 170° C., in particular from 110 to 160° C.

In the preferred extrusion process for producing the polyester film, themolten polyester material is extruded through a slot die and, in theform of a substantially amorphous prefilm, quenched on a chill roll.This film is then reheated and stretched longitudinally andtransversely, or transversely and longitudinally, or longitudinally,transversely, and again and longitudinally and/or transversely. Thestretching temperatures are generally from T_(G)+10° C. to T_(G)+60° C.(T_(G)=glass transition temperature), and the stretching ratio forlongitudinal stretching is usually from 2 to 6, in particular from 3 to4.5, and that for transverse stretching is from 2 to 5, in particularfrom 3 to 4.5, and that for any second longitudinal or transversestretching carried out is from 1.1 to 5. The first longitudinalstretching may, where appropriate, take place simultaneously withtransverse stretching (simultaneous stretching). Heat-setting of thefilm then follows with oven temperatures of from 180 to 260° C., inparticular from 220 to 250° C. The film is then cooled and wound.

The surprising combination of exceptional properties gives the film ofthe invention excellent suitability for a wide variety of applications,for example for interior decoration, for exhibition stands or exhibitionrequisites, as displays, for placards, for protective glazing ofmachinery or of vehicles, in the lighting sector, in the fitting-out ofshops or of stores, as a promotional item or laminating medium, forgreenhouses, for roofing systems, external cladding, protectivecoverings, applications in the construction sector, and illuminatedadvertising profiles, blinds, or electrical applications.

Its thermoformability makes the film of the invention suitable forthermoforming desired moldings for indoor or outdoor applications.

The invention is further illustrated below using examples.

The following standards or methods are used here in measuring theindividual properties.

TEST METHODS

DEG Content, PEG Content and IPA Content

DEG, PEG, or IPA content is determined by gas chromatography afterdissolving the thermoplastic polymer in cresol.

Surface Gloss

Surface gloss is measured at a measurement angle of 20° to DIN 67530.

Light Transmittance

Light transmittance is the ratio of the total transmitted light to theamount of incident light. Light transmittance is measured using the“®HAZEGARD plus” tester to ASTM D 1003.

Haze

Haze is that percentage proportion of transmitted light which deviatesby more than 2.5° from the average direction of the incident light beam.Clarity is determined at an angle of less than 2.5°.

Haze is [lacuna] using the “HAZEGARD plus” tester to ASTM D 1003.

Surface Defects

Surface defects are determined visually.

Mechanical Properties

Modulus of elasticity and tensile stress at break, and tensile strain atbreak, are measured longitudinally and transversely to ISO 527-1-2.

SV (DCA), IV (DVA)

Standard viscosity SV (DCA) is measured by a method based on DIN 53726in dichloroacetic acid.

Intrinsic viscosity (IV) is calculated from standard viscosity asfollows

IV (DCA)=6.67·10⁻⁴SV(DCA)+0.118

Fire Performance

Fire performance is determined to DIN 4102 Part 2, constructionmaterials class B2, and to DIN 4102 Part 1, construction materials classB1, and also to the UL 94 test.

Weathering (Bilateral), UV Resistance

UV resistance is tested as follows to the ISO 4892 test specification:

Tester Atlas Ci 65 Weather Ometer Test conditions Iso 4892, i.e.artificial weathering Irradiation time 1 000 hours (per side)Irradiation 0.5 W/m², 340 nm Temperature 63° C. Relative humidity 50%Xenon lamp internal and external filter made from borosilicateIrradiation cycles 102 minutes of UV light, then 18 minutes of UV lightwith water spray on the specimens, then again 102 minutes of UV light,etc.

Yellowness Index

(YI) is the deviation from the colorless condition in the “yellow”direction and is measured to DIN 6167. Yellowness indices (YIs) <5 arenot visually detectable.

In each case, the examples and comparative examples below use whitefilms of varying thickness, produced on the extrusion line described.

All of the films were weathered bilaterally to ISO 4892 testspecification, in each case for 1000 hours per side using the Atlas Ci65 Weather Ometer from the company Atlas, and then tested for mechanicalproperties, Yellowness Index (YI), surface defects, light transmission,and gloss.

Fire tests to DIN 4102, Part 2 and Part 1, and the UL 94 test, werecarried out on all of the films.

EXAMPLES Example 1

A white film of 50 m thickness is produced and comprises, as principalconstituent, polyethylene terephthalate, 7.0% by weight of titaniumdioxide, and 1.0% by weight of the UV stabilizer2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxyphenol (®Tinuvin 1577from the company Ciba-Geigy) and 2.0% by weight of flame retardant.

The titanium dioxide is of rutile type and has an average particlediameter of 0.20 μm, and has a coating of Al₂O₃. ®Tinuvin 1577 has amelting point of 149° C. and is thermally stable up to about 330° C.

For purposes of uniform distribution, the titanium dioxide and the UVabsorber is incorporated into the PET directly at the premises of thepolymer producer.

The flame retardant is the PET soluble organophosphorus compound AmgardP1045 from the company Albright & Wilson.

The flame retardant is fed in the form of a masterbatch. The masterbatchis composed of 10% by weight of flame retardant and 80% by weight ofPET, and its bulk density is 750 kg/m³.

The PET from which the film is produced and the PET that is utilized formasterbatch production have standard viscosity SV (DCA) of 810,corresponding to intrinsic viscosity IV (DCA) of 0.658 dl/g. DEG contentand PEG content are 1.6% by weight. 50% of the polyethyleneterephthalate, 30% by weight of recycled polyethylene terephthalatematerial, and 20% by weight of the masterbatch are charged at roomtemperature from separate feed vessels in a vacuum dryer which from thejuncture of charging to the end of the residence time traverses atemperature profile from 25 to 130° C. During the residence time ofabout 4 hours, the raw material mixture is stirred at 61 rpm.

The precrystallized or predried raw material mixture is post-dried inthe downstream hopper, likewise under vacuum, at 140° C. for 4 hours.The 50 μm monofilm is then produced using the extrusion processdescribed.

The individual steps of the process were:

Longitudinal Temperature: 85-135° C. stretching Longitudinal stretchingratio: 4.0:1 Transverse Temperature: 85-135° C. stretching Transversestretching ratio: 4.0:1 Setting Temperature: 230° C.

The white PET film produced had the following property profile:

Thickness 50 μm Surface gloss side 1 72 (Measurement angle 20°) side 268 Light transmittance 28% Surface defects per m² none Longitudinalmodulus of elasticity 3 700 N/mm² Transverse modulus of elasticity 4 800N/mm² Longitudinal tensile stress at break 130 N/mm² Transverse tensilestress at break 205 N/mm² Yellowness Index (YI) 48 Coloration uniform

The film fulfills the requirements of construction materials classes B2and B1 to DIN 4102 Part 2 and Part 1. The film passes the UL 94 test.

After 200 hours of heat-conditioning at 100° C. in a circulating-airdrying cabinet the mechanical properties are unaltered. The filmexhibits no embrittlement phenomena of any kind.

After in each case 1000 hours of weathering per side with the Atlas CI65

Weather Ometer the PET film has the following properties:

Thickness 50 μm Surface gloss side 1 65 (Measurement angle 20°) side 260 Light transmittance 35% Surface defects per m² none Longitudinalmodulus of elasticity 3 550 N/mm² Transverse modulus of elasticity 4 650N/mm² Longitudinal tensile stress at break 118 N/mm² Transverse tensilestress at break 190 N/mm² Yellowness Index (YI) 49

Example 2

Coextrusion technology is used to produce a multilayer PET film ofthickness 17 μm with the layer sequence A-B-A, B being the core layerand A being the outer layers. The thickness of the core layer is 15 μmand that of each of the two outer layers which cover the core layer is 1μm.

The polyethylene terephthalate used for the core layer B is identicalwith that of example 1 except that it comprises no UV absorber.

The core layer moreover comprises 2% by weight of flame retardant, theflame retardant being fed in the form of a masterbatch. The masterbatchis composed of 10% by weight of flame retardant and 90% by weight ofPET.

The PET of the outer layers has a standard viscosity SV (DCA) of 810 andhas been provided with 1% by weight of Tinuvin 1577 and 0.3% by weightof Sylobloc. The outer layers comprise no titanium dioxide and no flameretardant.

For the core layer, 50% by weight of polyethylene terephthalate, 30% byweight of recycled polyethylene terephthalate material, and 20% byweight of the masterbatch are precrystallized, predried, and post-driedas in example 1.

The outer layer polymer does not undergo any particular drying.Coextrusion technology is used to produce a film of thickness 17 μm withthe layer sequence A-B-A and with the following properties:

Layer structure A-B-A Total thickness 17 μm Surface gloss side 1 131(Measurement angle 20°) side 2 126 Light transmittance 49% Surfacedefects none (specks, orange peel, bubbles, . . . ) Longitudinal modulusof elasticity 3 550 N/mm² Transverse modulus of elasticity 4 130 N/mm²Longitudinal tensile stress at break 120 N/mm² Transverse tensile stressat break 155 N/mm² Yellowness Index (YI) 13.3 Coloration uniform

After 200 hours of heat-conditioning at 100° C. in a circulating-airdrying cabinet the mechanical properties are unaltered. The filmexhibits no embrittlement phenomena of any kind.

The film fulfills the requirements of construction materials class B2and B1 to DIN 4102 Part 2 and Part 1. The film passes the UL 94 test.

After in each case 1000 hours of weathering per side with the Atlas CI65 Weather Ometer the PET film has the following properties:

Layer structure A-B-A Total thickness 17 μm Surface gloss side 1 125(Measurement angle 20°) side 2 116 Light transmittance 45% Surfacedefects none (specks, orange peel, bubbles, . . . ) Longitudinal modulusof elasticity 3 460 N/mm² Transverse modulus of elasticity 4 050 N/mm²Longitudinal tensile stress at break 110 N/mm² Transverse tensile stressat break 145 N/mm² Yellowness Index (YI) 15.1 Coloration uniform

Example 3

A 20 μm A-B-A film is produced as in example 2, the thickness of thecore layer B being 16 μm and that of each of the outer layers A being 2μm.

The core layer B comprises only 5% by weight of the flame retardantmasterbatch of example 2.

The outer layers are identical with those of example 2, except that theycomprise 20% by weight of the flame retardant masterbatch used inexample 2 only for the core layer.

The raw materials and the masterbatch for the core layer and the outerlayers are precrystallized, predried, and postdried as in example 1.

The multilayer 20 μm film produced by means of coextrusion technologyhas the following property profile:

Layer structure A-B-A Total thickness 20 μm Surface gloss side 1 136(Measurement angle 20°) side 2 128 Light transmittance 41% Surfacedefects none (specks, orange peel, bubbles, . . . ) Longitudinal modulusof elasticity 3 400 N/mm² Transverse modulus of elasticity 4 100 N/mm²Longitudinal tensile stress at break 120 N/mm² Transverse tensile stressat break 160 N/mm² Yellowness Index (YI) 13.1

After 200 hours of heat-conditioning at 100° C. in a circulating-airdrying cabinet the mechanical properties are unaltered. The filmexhibits no embrittlement phenomena of any kind.

The film fulfills the requirements of construction materials classes B2and B1 to DIN 4102 Part 2 and Part 1. The film passes the UL 94 test.

After in each case 1000 hours of weathering per side with the Atlas CI65 Weather Ometer the PET film has the following properties:

Layer structure A-B-A Total thickness 20 μm Surface gloss side 1 124(Measurement angle 20°) side 2 117 Light transmittance 38% Surfacedefects none (specks, orange peel, bubbles, . . . ) Longitudinal modulusof elasticity 3 350 N/mm² Transverse modulus of elasticity 4 000 N/mm²Longitudinal tensile stress at break 105 N/mm² Transverse tensile stressat break 140 N/mm² Yellowness Index (YI) 15.8

Thermoformability

The films of examples 1 to 3 can be thermoformed on commerciallyavailable thermoforming machinery, e.g. from the company Illig, to givemoldings, without predrying. The reproduction of detail in the moldingsis excellent, with uniform surface.

Comparative example 1

Example 2 is repeated. However, the film is not provided with UVabsorbers, nor with flame retardant masterbatch. DEG content is thecommercially available 0.7%, and no PEG is present.

The white film produced has the following property profile:

Layer structure A-B-A Total thickness 17 μm Surface gloss side 1 139(Measurement angle 20°) side 2 130 Light transmittance 50% Surfacedefects none (specks, orange peel, bubbles, . . . ) Longitudinal modulusof elasticity 4 250 N/mm² Transverse modulus of elasticity 4 700 N/mm²Longitudinal tensile stress at break 180 N/mm² Transverse tensile stressat break 215 N/mm² Yellowness Index (YI) 12.0 Coloration uniform

The unmodified film does not fulfill the requirements of the tests toDIN 4102 Part 1 and Part 2, or of the UL 94 test.

The film has inadequate thermoformability.

After 1000 hours of weathering per side using the Atlas CI WeatherOmeter the film exhibits embrittlement phenomena and cracking on thesurfaces. This makes it impossible to measure the property profileprecisely—in particular the mechanical properties. Furthermore, the filmhas visible yellow coloration.

What is claimed is:
 1. A white, thermoformable film with a thickness inthe range from 1 to 350 μm, which comprises a crystallizablethermoplastic principal constituent, said thermoplastic having a DEGcontent of ≧1.0% by weight and/or a PEG content of ≧1.0% by weight, andcomprises at least one white pigment, said pigment being 0.5-25% byweight TiO₂ particles with a diameter from 0.10 to 0.50 μm or 1.0-25% byweight barium sulfate with a diameter from 0.20 to 1.20 μm, at least oneUV stabilizer, said stabilizer being 0.01-5.0% by weight of2-hydroxybenzotriazoles or triazines, and at least one flame retardantwhich flame retardant is soluble in the thermoplastic, said flameretardant being 1-20% by weight of dimethyl methyl phosphonate, and isfed directly during the production of the film by way of masterbatchtechnology, wherein the masterbatch has been pretreated by gradualheating at subatmospheric pressure, with stirring.
 2. The film asclaimed in claim 1, wherein the gradual heating at subatmosphericpressure, with stirring, is directly followed by post-drying at aconstant, elevated temperature, again at subatmospheric pressure.
 3. Thefilm as claimed in claim 1, wherein the UV stabilizer is selected fromone or more of 2(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxyphenol and2,2′-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethyl-butyl) phenol.
 4. Thefilm as claimed in claim 1, wherein the white pigment has a coating. 5.The film as claimed in claim 1, wherein the average particle size of theTiO₂ is from 0.10-0.30 μm.
 6. The film as claimed in claim 1, whereinthe surface gloss measured to DIN 67530 (measurement angle 20°) isgreater than
 15. 7. The film as claimed in claim 1, wherein the lighttransmittance measured to ASTM D 1003 is smaller than 70%.
 8. The filmas claimed in claim 1, wherein the modulus of elasticity measured to ISO527-1-2 is greater than 3200 N/mm² longitudinally and greater than 3500N/mm² transversely.
 9. The film as claimed in claim 1, wherein thecrystallizable thermoplastic is selected from polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate, andmixtures of one or more of these thermoplastics.
 10. The film as claimedin claim 9, wherein polyethylene terephthalate is used as crystallizablethermoplastic.
 11. The film as claimed in claim 10, which comprisesrecycled material.
 12. The film as claimed in claim 1, which has asingle-layer structure.
 13. The film as claimed in claim 1, which has amultilayer structure with at least one outer layer and with at least onecore layer.
 14. The film as claimed in claim 13, wherein the multilayerstructure has two outer layers and a core layer located between theouter layers.
 15. The film as claimed in claim 13 or 14, wherein atleast one UV stabilizer is present in the outer layer or layers.
 16. Thefilm as claimed in claim 13 or 14, wherein at least one white pigment ispresent in the base layer.
 17. The film as claimed in claim 13 or 14,wherein at least one flame retardant is present in the base layer.
 18. Aprocess for producing a thermoplastic film as claimed in claim 1, inwhich a crystallizable thermoplastic is melted in at least one extruder,and the resultant polymer melt corresponding to the composition of thefilm layer, or the resultant polymer welts corresponding to thecompositions of the outer and base layers, are fed to a die or,respectively, to a coextrusion die, and are extruded from the die onto achill roll, and the resultant prefilm is then biaxially oriented andheat-set, where the polymer melt for the base layer or for the outerlayer or layers or for the base layer and the outer layer or layerscomprise one or more of a flame retardant a white pigment, and thepolymer melt for the outer layer or layers comprise at least one UVstabilizer.
 19. The process as claimed in claim 18, wherein the additionof one or more of the flame retardant, the UV stabilizer and the whitepigment takes place by way of masterbatch technology.
 20. The method ofmaking an interior decoration, a display, a placard, a protectiveglazing, a shop outfit, a promotional item, a laminating medium, aroofing system, an external cladding, a protective covering, or anilluminated advertising profile or blind which comprise converting afilm as claimed in claim 1 into an interior decoration, a display, aplacard, a protective glazing, a shop outfit, a promotional item, alaminating medium, a rooting system, an external cladding, a protectivecovering, or an illuminated advertising profile or blind.